Dual-system hydraulic actuator

Abstract

An actuator includes a primary piston and a secondary piston axially movable together in a cylinder assembly; a plurality of chambers comprising: a first chamber, a second chamber, a third chamber, and a fourth chamber, wherein when fluid flows into the first chamber the primary piston extends and when fluid flows into the second chamber the primary piston retracts, wherein an extension fluid force applied by fluid in the third chamber to extend the secondary piston is different from a retraction fluid force applied by fluid in the fourth chamber to retract the secondary piston; and a force-balance chamber configured to receive fluid from the third chamber or the fourth chamber to apply a balancing force on the secondary piston to either oppose fluid force acting on the secondary piston or add to the fluid force acting on the secondary piston, thereby compensating for a difference between the extension fluid force and the retraction fluid force.

What is claimed is: 
     
       1. An actuator comprising:
 a primary piston that is disposed at least partially within a cylinder assembly; 
 a secondary piston that is disposed at least partially within the cylinder assembly, wherein the primary piston and the secondary piston are configured to move together axially within the cylinder assembly; 
 a plurality of chambers formed within the cylinder assembly and comprising: (i) a first chamber, wherein when fluid flows to the first chamber, the primary piston extends, (ii) a second chamber, wherein when fluid flows to the second chamber, the primary piston retracts, (iii) a third chamber, wherein when fluid flows to the third chamber, the secondary piston extends, (iv) a fourth chamber, wherein when fluid flows to the fourth chamber, the secondary piston retracts, and wherein an extension fluid force applied by fluid in the third chamber to extend the secondary piston is different from a retraction fluid force applied by fluid in the fourth chamber to retract the secondary piston; and 
 a force-balance chamber configured to receive fluid to apply a balancing force on the secondary piston, thereby compensating for a difference between the extension fluid force and the retraction fluid force. 
 
     
     
       2. The actuator of  claim 1 , wherein the cylinder assembly comprises:
 a primary cylinder; and 
 a secondary cylinder coupled to the primary cylinder, wherein the primary cylinder and the secondary cylinder form a longitudinal cylindrical cavity in which the primary piston and the secondary piston are axially-movable. 
 
     
     
       3. The actuator of  claim 2 , wherein the primary cylinder has a flange, wherein the secondary cylinder has a respective flange, and wherein the actuator further comprises:
 a central gland disposed between the flange of the primary cylinder and the respective flange of the secondary cylinder; and 
 a plurality of fasteners coupling the flange of the primary cylinder, the respective flange of the secondary cylinder, and the central gland to each other. 
 
     
     
       4. The actuator of  claim 3 , wherein the central gland comprises:
 a first gland portion interfacing with the flange of the primary cylinder; and 
 a second gland portion separate from the first gland portion and interfacing with the respective flange of the secondary cylinder. 
 
     
     
       5. The actuator of  claim 1 , further comprising:
 a piston rod disposed partially within the cylinder assembly and coupled to the secondary piston, wherein the primary piston is secured between the secondary piston and the piston rod, such that the primary piston, the secondary piston, and the piston rod move axially together. 
 
     
     
       6. The actuator of  claim 1 , further comprising:
 a static piston that is coupled to the cylinder assembly such that the static piston remains stationary; and 
 an inner cylinder that is coupled to the secondary piston and slidable about an exterior surface of the static piston, wherein the force-balance chamber is formed between the exterior surface of the static piston and an interior surface of the inner cylinder, such that the balancing force is applied to the inner cylinder, which transmits the balancing force to the secondary piston. 
 
     
     
       7. The actuator of  claim 6 , wherein the force-balance chamber is fluidly-coupled to the third chamber via a longitudinal channel and one or more cross-holes formed in the static piston, such that the force-balance chamber receives fluid from the third chamber as the secondary piston extends, wherein fluid in the force-balance chamber applies the balancing force on the secondary piston to oppose the extension fluid force applied to the secondary piston by fluid in the third chamber. 
     
     
       8. The actuator of  claim 6 , further comprising:
 a balance tube that is coupled to the static piston and interposed radially between the inner cylinder and the secondary piston, wherein the secondary piston is configured to move axially about an exterior surface of the balance tube as the secondary piston extends or retracts. 
 
     
     
       9. A method comprising:
 providing fluid to a first chamber and a third chamber of an actuator to extend a piston rod of the actuator, wherein the actuator comprises a primary piston and a secondary piston, wherein the secondary piston is coupled to the piston rod, and wherein the primary piston is secured to the secondary piston and the piston rod, such that the primary piston, the secondary piston, and the piston rod move together axially, wherein providing fluid to the first chamber causes the primary piston to extend, and wherein providing fluid to the third chamber causes an extension fluid force to be applied to extend the secondary piston; 
 providing fluid to a second chamber and a fourth chamber of the actuator to retract the piston rod of the actuator, wherein providing fluid to the second chamber causes the primary piston to retract, and wherein providing fluid to the fourth chamber causes a retraction fluid force to be applied to retract the secondary piston, wherein the extension fluid force is different from the retraction fluid force; and 
 providing fluid to a force-balance chamber to apply a balancing force on the secondary piston, thereby compensating for a difference between the extension fluid force and the retraction fluid force. 
 
     
     
       10. The method of  claim 9 , wherein the actuator further comprises: (i) a cylinder assembly in which the primary piston and the secondary piston are axially-movable, (ii) a static piston that is coupled to the cylinder assembly, such that the static piston remains stationary, and (iii) an inner cylinder that is coupled to the secondary piston and slidable about an exterior surface of the static piston, wherein applying the balancing force on the secondary piston comprises:
 applying the balancing force to the inner cylinder, which transmits the balancing force to the secondary piston. 
 
     
     
       11. The method of  claim 10 , wherein providing fluid to the force-balance chamber comprises:
 providing fluid from the third chamber through one or more cross-holes and a longitudinal channel formed in the static piston to the force-balance chamber as the secondary piston extends; and 
 applying the balancing force on the secondary piston to oppose the extension fluid force applied to the secondary piston by fluid in the third chamber.